[英]Get top n counted rows in table within group [MySQL]
我正在尝试仅获取按类别分组的前 3 种畅销产品(仅按每个类别的交易 (id) count(id)
出现的前 3 种产品)。 我一直在寻找可能的解决方案,但没有结果。 看起来它在 MySQL 中有点棘手,因为不能简单地使用 top() 函数等等。 示例数据结构如下:
+--------+------------+-----------+
| id |category_id | product_id|
+--------+------------+-----------+
| 1 | 10 | 32 |
| 2 | 10 | 34 |
| 3 | 10 | 32 |
| 4 | 10 | 21 |
| 5 | 10 | 100 |
| 6 | 7 | 101 |
| 7 | 7 | 39 |
| 8 | 7 | 41 |
| 9 | 7 | 39 |
+--------+------------+-----------+
如果您运行的是 MySQL 8.0,您可以使用窗口函数rank()
来实现:
select *
from (
select
category_id,
product_id,
count(*) cnt,
rank() over(partition by category_id order by count(*) desc) rn
from mytable
group by category_id, product_id
) t
where rn <= 3
在早期版本中,一种选择是使用相关子查询进行过滤:
select
category_id,
product_id,
count(*) cnt
from mytable t
group by category_id, product_id
having count(*) >= (
select count(*)
from mytable t1
where t1.category_id = t.category_id and t1.product_id = t.product_id
order by count(*) desc
limit 3, 1
)
在早期版本的 MySQL 中,我建议使用变量:
select cp.*
from (select cp.*,
(@rn := if(@c = category_id, @rn + 1,
if(@c := category_id, 1, 1)
)
) as rn
from (select category_id, product_id, count(*) as cnt
from mytable
group by category_id, product_id
order by category_id, count(*) desc
) cp cross join
(select @c := -1, @rn := 0) params
) cp
where rn <= 3;
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